Prerequisites

in most Linux distributions: the "Openoffice-Officebeans" package provided with your distribution

Download

Gnuaccounting is Free software under the GPL License: download here or via the Sourceforge info page. Please note that like any other Free software, Gnuaccounting comes without any warranty (see license with terms&conditions).
If you just want to try Gnuaccounting on Windows, there is also a ~300MB all-in-one-package including Java portable and OpenOffice.org portable which you can just can extract and run.